This invention relates to an aircraft stringer material and a method for producing the same.
Conventionally, AA7075 alloy is well known as a typical raw material for an aircraft stringer and has had widespread use in the aircraft field. Generally, the alloy is fabricated into the aircraft stringer by the following process.
The, AA7075 alloy ingot is homogenized by heating at about 460.degree. C. for 16 to 24 hours and hot rolled at 400.degree. C. to provide a plate approximately 4 to 8 mm thick. This plate is then intermediate annealed at around 410.degree. C. for 1 hour, cooled at a cooling rate not exceeding 30.degree. C./hr. and cold rolled to a plate of 2 to 4 mm in thickness. The cold rolled plate is annealed by heating to a temperature of 410.degree. C. for 8-12 hours and holding the temperature for about one to two hours. Further, the annealed plate is cooled to a temperature of approximately 230.degree. C. at a cooling rate of 30.degree. C./hr. or less, maintained at the temperature for 6 hours and air cooled to produce the primary stringer material.
Further the primary stringer material is subjected to a stepped cold working at various cold reductions ranging from 0 to 90%, and subsequently to a solution heat treatment, providing the secondary stringer material. The solution heat treatment may also be carried out without the stepped cold working.
Generally, the aforesaid primary material is known as O-material, while the secondary material as W-material. Thus, hereinafter the former is referred to as "O-material" and the later to as "W-material". W-material is formed into the desired shape, such as hat-shape, Z-shape or J-shape, by section roll forming and the treated W-material is subjected to a T6 temper treatment and, if required, further chemical milling to provide the aircraft stringer and stringer frame.
However, the conventional stringer materials have, for example, the following disadvantages:
The O-material produced from AA7075 alloy according to the above conventional method has a large grain size of 150-250 .mu.m and if the O-material is subjected to cold working with a relatively small amount of cold reduction, such as cold reduction of 10-30%, and then to the solution heat treatment, its grain size further grows, resulting in the reduction of mechanical properties and presenting many serious problems in the subsequent processes. Particularly, cold reduction of 20% is known to cause the marked grain growth.
Such considerable grain growth in the parts which are cold worked with a small amount of cold reduction causes reduction of elongation and fracture toughness, and leads to an undesirable orange peel appearance and occurence of cracks during section roll forming. Further, when material having a large grain size exceeding 100 .mu.m is subjected to a conventional chemical milling, which is practiced in the production of the aircraft stringer, the surface roughness increases remarkably and the fatigue strength is lowered. Hence, in case of the small amount of cold reduction, the production of the stringer is not only very difficult, but also the properties of the product are not satisfactory. In case a relatively large amount of cold reduction of more than 50% is followed by the solution heat treatment, it is possible to make the grain size approximately 50 .mu.m in the material. However, in practice, cold rolling reduction of a wide range of 0 to 90% is conducted on one piece of O-material of about 10 m in length so that it is extremely difficult to achieve a grain size not exceeding 100 .mu.m over the entire length.
Therefore, an O-material which can develop a fine grain size not exceeding 100 .mu.m in the W-material over the above wide range of cold reduction has been requested.
Still further, stringer materials prepared from a conventional Al-Zn-Mg-Cu alloy, such as AA7075 alloy, are generally inferior in corrosion-resistance, and thus highly improved corrosion-resistance is desired.